Electrochemical conversion of phenol to hydroquinone

ABSTRACT

The electrochemical conversion of phenol to hydroquinone or benzoquinone has been improved by increasing the ratio of hydroquinone to p-benzoquinone formed and the chemical efficiency of the process. This is accomplished by scoring or roughening the electrodes so that the planar surfaces contain notches having a depth and a width of about 0.1 millimeters up to about one-fifth the distance between the electrodes.

United States Patent Inventors Frank H. Covitz Lebanon; Robert V. Carrubba, Cranford, both of NJ. Appl. No. 12,865 Filed Feb. 19, 1970 Patented Oct. 26, 1971 Assignee Union Carbide Corporation New York, N.Y.

ELECTROCHEMICAL CONVERSION OF PIIENOL T0 HYDROQUINONE 4 Claims, No Drawings US. Cl 204/78, 204/280, 204/292 Int. Cl C0lb 29/06, BOlk 3/04, C07c 37/00 Field of Search 204/78,

[56] References Cited UNITED STATES PATENTS 1,055,652 3/1913 Reed 204/292 X 2,130,151 9/1938 Palfreeman 204/78 2,135,368 11/1938 Vagenius et al 204/78 3,509,031 4/1970 Covitz 204/78 Primary Examiner-F. C. Edmundson Attorneys-Paul A. Rose, Aldo .1. Cozzi and Bernard F Crowe ELECTROCHEMICAL CONVERSION OF PHENOL TO HYDROQUINONE BACKGROUND OF THE INVENTION This invention relates to the electrolysis of phenols and more particularly to the use of roughened or scored electrodes for improving the electrochemical conversion of phenol to hydroquinone.

A practical method of preparing hydroquinone has previously been developed which comprises the steps of:

a. electrolyzing an aqueous solution containing from about 0.5 to 4 percent by weight of a phenol and about I to 35 percent by weight of an electrolyte at a temperature of about 25 to 100 C., a pH of less than about 4, an anode DC potential of at least about 0.9 volts in reference to a saturated calomel electrode, a cathode DC potential more negative than +0.4 volts in reference to a saturated calomel electrode, and a current density of at least 4 amperes per square decimeter until up to about 80 percent by weight of the phenol has been electrolyzed to hydroquinone; and

b. recovering the hydroquinone from the aqueous solution.

In the above-described process the first step is the electrolytic oxidation of phenol to p-benzoquinone at the anode. The second step is the conversion of pbenzoquinone to hydroquinone by reduction at the cathode. Therefore during the electrolysis operation there is a certain ratio of hydroquinone to p-benzoquinone present in the reaction mixture. ln order for the preparation of hydroquinone in the abovedescribed process to be economically efficient it is desirable that the ratio of hydroquinone to p-benzoquinone in the effluent from the electrolysis cell be as high as possible since the unreduced pbenzoquinone must be either recycled or reduced in a separate step.

SUMMARY OF THE INVENTION It has now been found that the ratio of hydroquinone to pbenzoquinone in the electrolysis method described above may be raised by scoring or roughening the cathode surfaces imparting thereto notches or grooves having a depth and a width of at least about 0.1 millimeter up to about one-fifth the distance between the electrodes. The physical method of carrying out this roughening process is not narrowly critical and can be effected by grooving the cathodes by machining, engraving, embossing and the like. In its simplest form parallel lines are grooved across the face of the cathodes first lengthwise and then along the width of the electrodes. A common scribing tool or even a file can be used to produce this roughened effect.

The noncriticality of the means for effecting the scribing or roughening operation on the electrode surface is demonstrated by the fact that it has also been found quite convenient to employ a wire mesh screen in the range of about 8 to 200 mesh fabricated from a material of construction harder than that of the electrode. ln utilizing the wire mesh screen for roughening the electrode, the screen and electrode can be placed in a press and sufficient pressure applied to imprint the screen pattern on the surface of the electrode.

A second improvement in the general process delineated above for the electrolytic oxidation of phenol is provided by the practice of the instant invention in that the efficiency of the first step, that is, the oxidation of phenol to p-benzoquinone is increased by roughening the surface of the anodes used. This can be seen by an examination of the data in the examples infra demonstrating an increase in the yield and chemical efficiency of the electrolytic oxidation process.

While no critical limitations are imposed for the distance between the electrodes for the electrolytic oxidation of phenol to hydroquinone, practical considerations dictate that this distance should be about 0.1 to 2 centimeters in order to minimize the amount of electrical current used for maximum yield of products.

Similarly there are no critical limits for the distance between indentations or stated another way, the number of inill dentations per unit area of electrode surface. However, it is preferred to have as many indentations as possible per unit area of electrode surface limited only by the scribing method or tool. The limitations relating to the depth and width of the notches or indentations however are critical and are not arbitrarily chosen. For example, if the depth and width of these indentations on the electrodes are less than about 0.l millimeter, roughening or scoring would have little additional effect on the hydroquinone/p-benzoquinone ratio over that obtained with smooth electrodes even though the total microsurface area of the electrodes could conceivably be increased markedly. Although this invention is not limited to any particular theory or interpretation of the results, it is believed that this lower limit is dependent on diffusion phenomemon of substrate to and from the cathode surfaces and on the physical effect of gas bubbles which form in the indentations affecting this desired diffusion. As to the upper limit of about one-fifth the distance between the electrodes, this restriction is imposed by the fact that indentations larger than this value cause uneven current distribution between anode and cathode which in turn adversely affects the overall electrolytic process.

The improvement defined in this invention is applicable not only to phenol itself, that is C H,OH but also to ortho and meta substituted phenols such as the cresols and halogenated phenols.

The invention is further described in the examples which follow. All parts and percentages are by weight unless otherwise specified.

EXAMPLES l3 Control A The electrolysis cell was fabricated from a l.5 liter beaker having a'smooth lead anode l 1X7 cm.) centrally mounted in the beaker connected through a tab of lead at the top to an alligator clip joined to a heavy gauge copper wire. Two electrically common cathodes fabricated from l l 7 centimeter strips of smooth lead were mounted symmetrically in the beaker facing the anode on either side of the anode at a distance of approximately 3 centimeters. One liter of an aqueous solution containing 3 percent by weight of phenol and 3 percent by weight of sulfuric acid was placed in the beaker with the anode and cathode positioned below the surface of the solution. The solution was agitated by a magnetic stirrer rotating at the bottom of the beaker below the electrodes.

Three identical electrolysis cells were fabricated with the exception that the cathodes were scored to a depth and width of about 1 millimeter with a tile scribing parallel lines l centimeter, 0.5 centimeter, and 0.25 centimeter respectively apart first along the length of the cathodes and then along the width of the cathodes. The four cells were then connected in series and about 20 ampere-hours at a current density of about 20 amperes per square decimeter was then passed through the series of cells at a temperature of about 40 to 45 C. After minutes samples were taken from each of the 4 cells and the amount of hydroquinone and quinone determined by standard polarographic techniques. The results observed were tabulated table I below:

cathode efficiency with the scored or roughened cathodes over the smooth control.

EXAMPLES 4 and 5 Control B duplicated A except for current density. The procedure described for examples 1-3 was repeated in examples 4 and 5 with the exception that the cathodes were allowed to remain smooth and the anodes were scored with a file to a depth and width of about 1 millimeter at intervals of 0.5 and 0.25 centimeter respectively. The three cells for control B and examples 4 and 5 were also connected in series but the electrolysis was run at a current density of 40 amperes per square decimeter for about 60 minutes. After this time samples were taken and the hydroquinone and p-benzoquinone measured as before. The data thus obtained are presented in table ll below:

These data indicate that there is about a 6 percent increase in the efficiency of scored anodes, that is, in the production of hydroquinone.

EXAMPLES 6-8 The procedure described in examples 1-3 was followed with the exception that the current density was 40 amperes per square decimeter and the amount of current was amperehours Control C employed smooth cathodes and anodes. Example 6 was the same as example 5, that is, smooth cathodes were used with an anode scored to depth and width of about 1 millimeter at intervals of 0.25 centimeter. Example 7 employed cathodes scored by pressing a 16 mesh steel screen against both planar surfaces of the cathodes providing indentations of about 0.75 millimeter deep and about 1.5 millimeters wide. Example 8 employed cathodes scored by impressing a 32 mesh steel screen against both planar surfaces providing depressions 0.38 millimeter deep and 0.75 millimeter wide. The results obtained are presented in table 111 below:

moles grams moles grams Control C 0.074 8.0 0.033 3.5 33.4 6 0.088 9.5 0.033 3.5 37.8 7 0.087 9.4 0.033 3.5 37.8 8 0.094 10.2 0.030 3.2 38.8

ANALYTICAL DETERMINATION OF HYDROQUINONE AND p-BENZOQUINONE One milliliter samples were withdrawn from the electrolysis cells and transferred to a 25 ml. volumetric flask and the liquid meniscus brought to the fiducial mark with a 0.2 molar pH, aqueous phosphate buffer. A polarogram of this solution was obtained and the diffusion limiting current for hydroquinone p-benzoquinone was determined. These data were compared with calibration curves prepared from standard hydroquinone and p-benzoquinone solutions. The calibration curves consisted of a plot of diffusion limiting current in microamperes versus concentration in moles or ggams.

Although the invention has een described w|th some degree of particularity, it is understood that many changes and modifications can be made without departing from the spirit and scope of the invention.

What is claimed is:

1. 1n the method of preparing hydroquinone which comprises the steps of:

a. electrolyzing an aqueous solution containing from about 0.5 to 4 percent by weight of phenol and about 1 to about 35 percent by weight of an electrolyte at a temperature of about 25 to 100 C., a pH of less than about 4, an anode DC potential of at least about +0.9 volts in reference to a saturated calomel electrode, a cathode DC potential more negative than +0.4 volts in reference to a saturated calomel electrode, and a current density of at least 4 amperes per square decimeter until up to about percent by weight of the phenol has been electrolyzed to hydroquinone; and

b. recovering the hydroquinone from the aqueous solution,

the improvement which comprises using lead electrodes from 0.] to 2.5 cm. apart, scoring the electrode surfaces to provide a series of continuous notches to a depth and a width of about 0.1 millimeter up to one-fifth the distance between the electrodes, whereby the ratio of hydroquinone to p-benzoquinone is increased.

2. Method claimed in claim 1 wherein the scoring is first effected in a series of parallel lines along the length of the electrodes and then in a series of parallel lines normal to said first said of parallel lines along the width of the electrode.

3. Method claimed in claim 1 wherein the scoring is achieved by pressing a metal wire screen into the surface of the electrodes.

4. Method claimed in claim 3 wherein the metal screen is about 8 to 200 mesh. 

2. Method claimed in claim 1 wherein the scoring is first effected in a series of parallel lines along the length of the electrodes and then in a series of parallel lines normal to said first said of parallel lines along the width of the electrode.
 3. Method claimed in claim 1 wherein the scoring is achieved by pressing a metal wire screen into the surface of the electrodes.
 4. Method claimed in claim 3 wherein the metal screen is about 8 to 200 mesh. 